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Introduction to Ceramics Processing Jürgen G. Heinrich Cynthia M. Gomes L e c tu re M a n u s c ri p t 1 2 1 22000 e N I k T A B Authors Jürgen G. Heinrich is Professor of Engineering Ceramics at the Institute of Non-metallic Materials at Clausthal University of Technology, Germany. He received a B.Sc. degree in materials engineering from the Johann Friedrich Böttger Institute in Selb, Germany, a M.Sc. and a Ph.D. in materials science both from the Technical University in Berlin, Germany. He joined the faculty of Materials Science at Clausthal University of Technology after several years of research activity at the German Aerospace Center in Cologne and at Hoechst CeramTec AG in Selb, Germany. He was president of the German and the European Ceramic Society and secretary of the International Ceramic Foundation. He is fellow of the American and the European Ceramic Society, editor of the Journal of Ceramic Science and Technology and Senior Visiting Professor of the Chinese Academy of Sciences. Cynthia M. Gomes is research scientist at the BAM, Federal Institute for Materials Research and Testing in Berlin, Germany. She received her Diploma in Materials Science and Engineering from the Federal University of Paraiba, Brazil, with M.Sc. and Ph.D. in Materials Science and Engineering both from the Federal University Santa Catarina, Brazil. After working as guest visitor scientist and a two-year post doctorate at the University of Erlangen-Nuremberg she has joined the BAM. At the Division for Ceramic Processing and Biomaterials she has been working mainly in the field of additive manufacturing of ceramic materials, co-working also on national and international (DIN, ISO) groups for standardization of these technologies. Acknowledgement For a better understanding of the theory video clips and computer animations are available in this lecture manuscript. The videos have been shot at different ceramic manufacturers and equipment as well as raw material suppliers by the cameraman of Clausthal University of Technology, Stefan Zimmer. We would like to take the opportunity to thank him for his professional work and excellent performance and the following companies for their assistance: ARBURG GmbH & Co KG, Loßburg, Germany BHS tabletop AG, Weiden, Germany CeramTec GmbH, Marktredwitz, Germany Engineered Pressure Systems International N.V. Oost-Vlaanderen,Belgium Imerys Tableware GmbH, Tirschenreuth, Germany Riedhammer GmbH, Nürnberg, Germany Rosenthal GmbH, Selb, Germany SGL Carbon GmbH, Meitingen, Germany Notes For an optimum presentation of the PDF-file please use the current Adobe Reader version: www.adobe.com/products/reader.html The videoclips are available in different formats (mov, avi and mp4). To watch the videos the Adobe Flash Player (www.adobe.com/support/flashplayer/downloads.html) is an appropriate player for the most operating systems. If you cannot open the videos in the lecture manuscript please find them at video.tu-clausthal.de/film/435.html Contents 1. Introduction .......................................................................................................... 3 2. Raw materials ............................................................................................................ 14 2.1 Structure of the earth ......................................................................................... 14 2.2 Deposits ........................................................................................................ 24 2.3 Natural ceramic raw materials ............................................................................... 31 2.3.1 Kaolins and clays ......................................................................................... 31 2.3.2 Feldspars ..................................................................................................... 37 2.3.3 Quartzites and sands ................................................................................... 39 2.3.4 Binary and ternary silicates, high alumina containing raw materials ............. 44 2.4 Synthetic ceramics raw materials .......................................................................... 48 2.4.1 Silicates ....................................................................................................... 48 2.4.2 Oxides ......................................................................................................... 51 2.4.3 Non-oxide materials ..................................................................................... 54 2.5 Organic raw materials ........................................................................................... 56 2.6 Raw material preparation ............................................................................... 67 3. Body preparation ......................................................................................................... 80 3.1 Grain size modification .................................................................................. 80 3.2 Classification ....................................................................................................... 93 4. Forming ...................................................................................................................... 100 4.1 Introduction ......................................................................................................... 100 4.2 General principles ............................................................................................... 100 4.2.1 Characterisation of suspensions ................................................................ 101 4.2.1.1 Particle charging in liquid suspensions ............................................. 101 4.2.1.2 Electrical double layers on particle surfaces …………………………...105 4.2.1.3 Electrokinetic properties and slip stability .................................... 107 4.2.1.4 Rheological properties of ceramic suspensions ................................ 113 4.2.2 Plasticity of ceramic systems ..................................................................... 118 4.2.3 Granulation ................................................................................................ 122 4.2.3.1 Production of granules ............................................................... 123 4.2.3.2 Characterization of granules ............................................................. 129 4.3 Forming .......................................................................................................... 132 4.3.1 Casting processes ..................................................................................... 134 4.3.1.1 Slip casting ................................................................................ 134 4.3.1.2 Pressure casting ............................................................................... 137 4.3.1.3 Tape casting .............................................................................. 139 4.3.2 Plastic deformation forming ....................................................................... 148 4.3.2.1 Roller tool forming … ..................................................................... 148 4.3.2.2 Extrusion ................................................................................... 150 4.3.2.3 Injection molding … ....................................................................... 154 4.3.3 Pressing .................................................................................................... 1624.3.3.1 Uniaxial dry pressing ................................................................. 162 4.3.3.2 Isostatic Pressing .............................................................. 166 4.3.4 New developments .............................................................................. 169 4.4 Literature list of chapter "Forming" ...................................................................... 174 5. Thermal processes .................................................................................................... 180 5.1 Drying .......................................................................................................... 180 5.2 Sintering .......................................................................................................... 186 5.2.1 Sintering of porcelain, silicate and oxide ceramics ..................... 186 5.2.2 Sintering of non-oxide ceramics ......................................................... 195 5.2.3 Pressure sintering ...................................................................................... 197 5.2.4 Microwave sintering ................................................................................... 198 6. Finishing and post processing ................................................................................. 201 6.1 Glazing ........................................................................................................ 201 6.2 Decoration .......................................................................................................... 210 7. Special technologies ................................................................................................. 214 7.1 Porcelain production ........................................................................................... 214 7.2 Brick production .................................................................................................. 216 7.3 SiC production .................................................................................................... 216 7.4 Piezoceramics production ................................................................................... 217 7.5 Laser sintering ............................................................................................. 218 8. Literature .................................................................................................................... 219 3 1. Introduction The term „Ceramics Processing“ describes the process of production of ceramic components from natural to synthetic raw materials as well as their disposal. Contrary to metals, polymers or glasses, the starting materials for the production of ceramic materials are powders. These powders are brought into shape and the components then are sintered which is a temperature treatment clearly below the melting point. This technique is applied because of the high melting points of ceramic materials which make casting impossible or uneconomical. The starting material can be of oxidic or non-oxidic nature; some care must be taken in order not to sinter both types of materials in the same furnace. Example: silicon nitride would incinerate or combust if sintered in an oxidizing atmosphere. Therefore, furnace technology for non-oxidic materials must be different from the one for oxidic starting materials. This is the reason why this lecture “Ceramics Processing” can only give an overview of the most important materials. The variety of technological procedures is so large that it is not possible to describe everything in detail. Classification of the most important material groups distinguishes between natural and synthetic materials. Natural raw materials are extracted from earth, and these raw materials must be further processed. They are blast in quarries, i.e., pieces of rock are exploited and worked up to powders (materials) (Fig. 1.1). From these materials pre-products are produced by forming or shaping. Metal components can be formed during the process of reshaping whereas ceramic components can be produced only by a sintering process. The product in later time has to be disposed by recycling or remineralisation. Fig. 1.1: Cycle of materials by Ondracek. 4 Synthetic materials are classified into non-metallic materials, semiconductors and metallic materials (Fig. 1.2). Non-metallic materials are non-conductors, or insulating materials. Metallic materials have a very high electric conductivity; in semiconductors the electric conductivity can be found in between. Non-metallic materials are devided into inorganic and organic materials. Oxide and non-oxide ceramics as well as glass belong to the group of inorganic materials. Fig. 1.2: Classification of the main material groups. To modify their characteristics ceramic materials are often treated together with organic or metallic materials, originating the category of composite materials. From the chemical point of view, ceramic materials can be divided into oxides and non-oxides (Fig.1.3). Oxidic ceramics can be made of natural or synthetic raw materials. 5 Fig. 1.3: Classification of ceramic materials. Non-oxide ceramic materials also made of synthetic raw materials are classified into carbides, nitrides, borides, silicides. The complete group of metal oxides within the periodic system belongs to ceramic materials, mostly made from synthetic raw materials. Silicates are made from natural raw materials. In particular within industry, another classification has been established which will be discussed on the next topics. 6 Silicates are often divided into coarse clay ceramics and fine ceramics, subdivided into their constituents (>1 mm coarse clay ceramics, <1 mm fine ceramics). Further distinction is made between porous and dense materials and, depending on the degree of purity of the raw materials, for example brightly burning porcelain (almost white) and coloured materials such as tiles and bricks (Fig 1.4). Fig. 1.4: Classification of ceramic materials [1]. Another overview of silicates can be seen from the ternary phase diagram of clay or kaolin, quartz and feldspar (Fig. 1.5). Porcelain is a mixture of kaolin, feldspar and quartz. It is situated approximately in the middle of this diagram. Stoneware and earthenware can also be found here. So, it must have be taken into consideration that a wide range of materials with different technological production processes, therefore various procedures, from the starting powder to the final product is necessary. 7 Fig. 1.5: Diagram with different ceramic compositions from the system clay or caolin- feldspar-quartz in dependence of the temperature [1]. Fig. 1.6 elucidates the reason why ceramic materials are made of powders, which after shaping must undergo a sintering process to achieve the final properties and, unlike metallic materials, cannot be molten and cast in a mould. This is mainly due to the high melting temperatures of these materials, often above 2000°C. Technologically, it is extremely difficult to produce molten masses at such high temperatures and cast them into suitable containers. In Fig. 1.6 another difference of the ceramics with regard to metals can be seen. Due to the nature of the covalent or ionic bonds in the ceramic materials, their electron conductivity in is quasi equal to zero. Ionic conductivity is extremely low respectively the specific electric resistance is very high. 8 Fig. 1.6: Properties of high-melting oxides [1]. Uranium compounds with their high densityalso belong to the ceramic materials (Fig. 1.7). Here again, a particular technology, for example for the production of nuclear fuel rods is required. Fig. 1.7: Properties of some Uranium compounds [1]. 9 Refractories are another material group (Fig. 1.8). Mixed oxides of silicon oxide, alumina, chromium oxide and magnesium oxide are part of this group as well as refractory bricks and chrome-magnesia stones. Refractories present a very high temperature resistivity and a very good corrosion resistance and are used in steel, binder or glass industry for kiln lining, thermal insulators. Fig.1.8: Softening temperature of some refractories under load [1]. Non-oxidic ceramic materials (Fig. 1.9) have extremely high melting temperatures (over 3000°C) and a very low density. Today, these materials are used for machinery construction or electronics, for example, when the emphasis is to achieve low thermal conductivity and low specific weight. Movable and abrasion-resistant components are increasingly applied for automobile fabrication or aerospace industry where high temperature-resistant materials with low specific weight and therefore low inertia masses are needed. 10 a) sublimation Fig. 1.9: Properties of some non-oxidic substances [1]. Fig. 1.9 presents some materials which are not classified among ceramics, for example, titanium carbide, zirconium carbide, and titanium nitride or zirconium boride. Despite the fact they are produced like ceramics, they show metallic bonds, which means electron conductivity. Composite materials with different matrixes and reinforcement components have been developed in order to combine the advantages of the different material categories (Fig.1.10). 11 Fig. 1.10: Classification of composites. This variety of materials requires various production technologies. Using the example of a carbon brake disc assembled in some classy cars, the following video clip shows how complex production technology may be. Videoclip: Processing of carbon brake discs Carbon fibres mixed with carbon powder and organic additives are the starting material for the production of ceramic brake disks. This fibre-powder-mixture is first put into shape. The shaping procedure used here is called uniaxial dry pressing. It is difficult to automate processing of fibre materials, therefore production is mostly manually: A plastic model is placed into the mould. This will be later burned out; the plastic model geometry then generates the cooling channels for the brake disk. Uniaxial pressing is made at slightly increased temperatures in order to liquefy the polymer material and facilitate consolidation. This almost manual production process makes also clear why brake disks have such a high price. A set for the Porsche Carera costs about 7,500 Euro. The organic additives have to be burned out after mould release. This is made in an inert gas atmosphere where the plastic materials are cracked and the carbon relicts remain in the structure. 12 Burning out of the organic additives has to be made very carefully so that no cracks are formed, because it is related to volume expansion. Gas feeding and gas evacuation must be extremely well-controlled in order to keep atmosphere constant. After this, the carbon brake disk presents a machineable state – like e.g. graphite. However, it is porous and has not yet the resistance needed for use in a car. This condition is perfect for conventional treatment like boring a hole, cutting, combining the cooling channels. After this green machining the carbon disk is infiltrated with silicon (element). For this purpose, the disks lying on Si powder are put into a vacuum furnace. Temperature is set above the silicon’s melting point – to about 1,500°C, when silicon enters the pore channels. Part of the silicon reacts with the carbon to build SiC; part of it remains as silicon. So the brake disk consists of carbon fibre, silicon carbide and free silicon. The technological processes of porcelain production or fabrication of alumina substrates for electronics are different. Therefore it will not be possible to describe in this lecture every procedural step, but the most important procedures in ceramic production can be demonstrated. This brings us to this lecture’s outline: We will first take a look on natural raw materials, later on synthetic and organic materials needed for shaping. The structure of earth elements and deposits will be discussed in the context with natural raw materials. Ceramic raw materials have to be prepared for further processing. When quarried out from a mine they often appear as pieces of rocks. These have to be milled to a desirable grain size. We will talk about this in chapter “Processing of raw materials”. After milling, separating and fractionating the raw materials get normally mixed into masses which are no longer subject to natural fluctuations, but show uniquely defined profile properties. Once the raw materials or masses are accordingly prepared, this must be shaped. 95 % of the ceramic powders get in contact with water during shaping. As a start, we will therefore concentrate on basic theories, dedicating to the question what happens if powder is dispersed in water. What happens on the particles’ surface and how can we modify this surface? The intention is reduction of the water content of such suspensions (ceramicists call it slurry), because the water has to be removed before sintering starts, and every kilogram of water which has to be evaporated, costs money. 13 We will then discuss different forming technologies and differentiate, in particular, fluid, malleable and dry procedures. Shape forming will be followed by thermal treatment. Water is evaporated during the drying process; furthermore organic additives will be burned out. The following sintering process is related to a treatment temperature below their melting temperature. We will first talk about theoretical basics and then specifically about the silicate ceramic’s firing, about sintering of oxide and non-oxide ceramic materials and sintering at elevated pressure. At the end of this lecture we will discuss finishing processes and further treatment of ceramic materials. With regard to silicate ceramic materials (e.g. porcelain) this means glazing and decoration. As for technical ceramic materials cutting, polishing or coating is concerned. At the end of these lectures I will once again pick up some typical examples regarding this variety of particular technologies and compare procedures at porcelain and brick factories or the production of piezoceramics and silicon carbide. 14 2. Raw materials 2.1 Structure of the earth We first have to learn where the rocks come from which are the basis of ceramic raw materials. Then we can understand why these raw materials’ chemical composition is fluctuating and what we can do to stop those fluctuations during preparation. Fig. 2.1.1 shows schematically the sectional view through the terrestrial body. The inner section consists of an iron-nickel core with a radius of 3,500 km. This section is called barysphere with a specific weight of 9.6 g/cm³. The oxide-sulphide interlayer has a thickness of 1,700 km. We call this layer chalkosphere with a specific weight of 6.4 g/cm³. Lithosphere has a thickness of 1,200 km with a specific weight of 3.4 g/cm³. The crust of the earth with just a few kilometres thickness has a specific weight of 2.7 g/cm³. Fig. 2.1.1: Schematic cross section through earth by Suess-Wiechert[2]. The chemical composition of this earth crust (Fig. 2.1.2) is important for the raw materials’ chemical composition. The earth crust consists mostly of oxides. The most important ones are SiO2 and Al2O3. Iron oxide is often regarded as a contamination in the raw materials, which are commonly undesired once it gives the end product a strong red colour. Magnesium oxide, calcium oxide, sodium and potassium oxides are other important components in natural raw materials used for the preparation of ceramic products. 15 Fig. 2.1.2: Average Chemical Composition of Earth Crust up to 16 km Depth from Barth, Correns, Eskola [2]. The interior of the earth consists of liquid magma. When it approaches the surface of the earth the magma solidifies. Igneous rocks have a so-called calcium-alkaline line and the so- called sodium and potassium alkaline earth lines. With regard to slow solidification we talk about plutonic rocks, such as granite. If magma erupts from a volcano and solidifies very quickly, we talk, for example, about quartz porphyries, basalt or diabase, according to their chemical composition (Fig. 2.1.3). Fig. 2.1.3: Geological Tree of volcanic Rocks by Cloos [2]. 16 Chemical composition for example of granite and quartz porphyry is the same, their crystallite size is different. Deep inside earth, granite had a long time to crystallise out (big grain size), however, quartz porphyry quickly petrified on the surface (small grain size). In between there are the so-called dyke rocks, like e.g. quartz. Fig. 2.1.4 describes the types of rocks and their mineralogical composition. Chemical composition for typical rocks can be found in Fig. 2.1.5. Fig. 2.1.4: System of magmatic stones with their mineral composition. Fig. 2.1.5: Chemical composition of stones [2]. 17 Fig. 2.1.6 shows the formation of volcanites. Liquid magma is brought very quickly to the earth’s surface, crystallising into very fine grains. There is no significant change in its chemical composition. Secondary sites can develop from these primary ones. Rocks can mechanically weather or chemically change (Fig. 2.1.7), producing chemical or biogenic sediments. Mechanical weathering brings water into the rocks, which freezes during winter. Stresses occur and the material weathers (Fig. 2.1.8). A rock unit may also weather when mineral grains are removed in saline solutions (Fig. 2.1.9). Mechanical sediments are formed following mechanical weathering and transport by water or air, chemical or biogenic sediments follow chemical weathering (Fig. 2.1.7). This weathered rock in Southern Taiwan clearly shows that the processes schematically outlined in nature really occur (Fig. 2.1.10). Fig. 2.1.6: Formation of Vulcanite. 18 Fig. 2.1.7: Scheme of rock metamorphism via alteration [1]. Fig. 2.1.8: Schematic representation of physical weathering of rocks with multi-mineral components. 19 Fig. 2.1.9: Dissolution of mineral grains from rock formation via salt weathering. Fig. 2.1.10: Dissolution of mineral grain from rock formation (South Taiwan, Nov. 2007). Kaolin develops as a result of weathering of feldspar. Feldspar is a potassium-aluminium- silicate. There are two different possibilities of weathering: The so-called allitic weathering means removal of some of the components from the feldspar structure during millions of years, with aluminium hydroxide remaining. Siallitic decomposition means that K2O and water leave the system and mineral remains which we call kaolin (Fig. 2.1.11). Weathering products can be transported to secondary mineral deposits by water or air (Fig. 2.1.12 and 20 2.1.13). When kaolin is transported and found on secondary deposits, we call it clay. Clay is much finer than kaolin, because due to the transport the coarse crystals remain further up, the fine ones further down. However, it is more contaminated since in this way metal and organic contaminations increase. Transport to the secondary deposits may cause different sediment structures (Fig. 2.1.14). When sediments are transported by earth faults into deeper regions and come again under pressure, we get, for example, mudstone (Fig 2.1.15), lime stone (Fig. 2.1.16), marble or quartzite (Fig. 2.1.17). This hardening is called diagenesis, which may also cause chemical changes (Fig. 2.1.15 left). Fig 2.1.18 recapitulates the formation of rocks. Fig. 2.1.11: Allitic and siallitic weathering of potassic feldspar. Fig. 2.1.12: Formation of sediments. 21 Fig. 2.1.13: Transport and deposition of clastic material. Fig. 2.1.14: Type of sediments: (left) sediments in water – layered -; (right) sediments in glacier [Moraine] – unlayered. Fig. 2.1.15: Diagenesis of clay into clay stone. 22 Fig. 2.1.16: Diagenesis of sand and lime slurry. 23 Fig. 2.1.17: Metamorphosis of lime stone into silica sandstone. Fig. 2.1.18: Simplified scheme of rock formation by Kukuk [2]. 24 2.2 Deposits The feldspar deposit from Fig. 2.2.1 shows besides clear feldspar sections a quartz stock and pegmatite sections. The mining waste has to be removed first before exploitation can start. Feldspars exist in three pure forms: potassic feldspar, albites and anorthite. In nature there are no mixed deposits of anorthites and potassic feldspars. But deposits of albites and anorthites, as well of sodium and potassic feldspar can naturally occur (Fig. 2.2.2). Fig. 2.2.1: Pegmatit from Hagendorf (Oberpfalz) [2]. Fig. 2.2.2: Triangular phase diagram (ternary system) Orthoclase – Albite – Anorthite; Variations in the chemical composition of natural feldspars (according to Betechtin). 25 No. Origin SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Loss on ignition SK (PCE) % % % % % % % 1 Germany Siebengebirge Wintermühlenhof 97,46 0,46 1,00 traces traces 1,13 -- 2 Westerwald Herschbach 97,94 0,92 0,19 traces traces 0,18 >34 3 Vogelsberg Rainrod 98,42 0,58 0,19 0,20 0,19 0,50 34 4 Hessische Senke Marienrode 97,30 0,65 0,84 0,45 traces traces 0,39 >34 5 Süd-Hannover Kattenbühl 98,30 0,17 0,64 0,69 0,07 traces 0,15 >34 6 Sachsen Glossen 98,39 0,64 0,32 0,20 traces 0,25 34 7 Italy 97,90 0,36 0,64 0,06 traces traces 0,85 -- 8 South Africa 94,20 0,29 1,72 0,76 2,01 traces -- -- Fig. 2.2.3: Analysis of different concrete quartzites. Quartz is won in quartz dikes. This raw material, as for feldspars, requires intensive milling. Quartz deposits have a relatively high purity (Fig. 2.2.3), as there are quartz deposits with a SiO2 content >99%. Such raw materials are especially used for the production of glass where pure SiO2 grades are particularly demanded. Fig. 2.2.4 shows schematically a typical profile of a kaolin deposit. Kaolins developed during weathering are normally not much pure. Clays are often found on secondary deposits together with brown coal beds. Faults of earth occurred during millions of years cause problems to the exploitation of raw materials. Due to earth faults, layers with different chemical compositions lay next to each other. Therefore, much care has to be taken with regard to the preparation of such raw materials in order to either compensate these differences in chemical compositions by mixing or separate them from each other. 26 Fig. 2.2.4: Deposit profiles [3]. Fig. 2.2.5: SEM (Scanning Electron Microscopy) of clay minerals [2]. 27Grain size of weathered kaolin is extremely small (Fig. 2.2.5). So this raw material does not have to be further crushed. But it has to be dispersed, suspended into water. Halloysite has a kaolin structure with increased water content. This mineral shows small hexagonal kaolinite plates composed of different layers, which curls, since the size of the layers varies and chemical bonds between the layers are only possible following geometric deformation. This means that the raw material cannot be used for the processing of components because its transport in tubes for fully automated industrial facilities is extremely difficult. The chemical composition of pure kaolin is: 46.6 weight percent (wt. %) SiO2 – 39.5 wt. % Al2O3 and 13 wt. % water (Fig. 2.2.6). Its water content can be noticed in Fig. 2.2.6 as loss of ignition, which happens when kaolin undergoes thermal treatment, also see detailed reviewed in section “Sintering”. SiO2 % Al2O3 % TiO2 % Fe2O3 % CaO % MgO % Alkalis % LOI. % SK (PCE) pure kaolonite prime quality 46,6 46,25 39,50 39,28 0,14 0,64 0,14 traces 0,15 13,9 13,40 36 Kaolin from Seilitz 56,49 30,66 0,57 0,25 0,30 0,96 10,84 -- Kaolin from Kemmlitz 56,47 30,58 0,81 0,11 0,06 0,64 11,39 34 Kaolin from Gösen 52,95 32,65 1,27 0,09 0,61 1,55 10,96 34 Schnaittenbach's raw kaolin 84,58 11,32 0,52 -- -- 0,24 3,34 33/34 Fig. 2.2.6: Chemical analysis of some kaolins. As already mentioned clay is more contaminated than kaolin due to transport on secondary deposits (Fig. 2.2.7). With regard to sintered products the high content of iron-oxide causes massive red coloration (clay bricks). 28 SiO2 % Al2O3 % TiO2 % Fe2O3 % CaO % MgO % Alkalis % LOI. % SK (PCE) Blue clay from Saarau 47,9 36,4 1,8 0,8 0,4 n. b. 12,3 34 Blue clay from Rauske 50,4 34,8 2,1 0,6 0,5 n. b. 11,4 34/35 Pot clay from Wiesau 51,3 33,7 1,8 0,9 0,5 2,2 9,7 32 Premium clay from Zinzendorf 47,6 36,8 1,8 0,03 Spur 0,3 13,4 34 Clay from Groß- Saubernitz 57,0 28,0 3,5 -- 0,6 1,9 9,2 31 Clay from Schwepnitz 48,8 35,3 1,9 0,03 -- 1,0 12,9 34 Premium clay from Hohenbocka 48,8 34,0 1,3 0,15 0,3 1,8 13,6 33 Fat clay from Großalmerode 48,1 35,1 1,4 2,2 -- -- n.b. 12,3 33 Fig. 2.2.7: Analysis of some selesian, middle German and hessian clays. With regard to the shifting of rocks the sectional drawing of a magnesite deposit (Fig. 2.2.8) makes clear how many efforts are required to exploit these raw materials. Magnesite is used for the production of refractory materials for the steel, cement or the glass industry. 29 Fig. 2.2.8: Geological Profile through magnesit deposit [3]. Zirconium is a natural zirconium silicate, so a solid solution from zirconium oxide and SiO2. SiO2 % ZrO2 % Al2O3 % TiO2 % Fe2O3 % CaO % MgO % Alkalis % LOI. % Zircon from Madagascar 33 66 - Zircon from Ceylon 33,86 64,25 - - 1,08 - - - - Sand from Florida, cleaned - - - - Brazilian Baddeleyite 0,70 96,52 0,43 - 0,41 0,55 0,10 0,42 0,39 Zircon-favas 0,48 97,19 0,40 0,48 0,92 Spur - - 0,38 Zircon-favas, light brown 15,35 81,64 0,90 0,51 1,10 - - - 0,63 1 98,5 1,5 Fig. 2.2.9: Analysis of some zirconium raw materials [3]. Raw materials with a high percentage of high alumina are highly attractive materials for the production of ceramic components for the electronics and mechanical engineering industries and will therefore be separately considered (Fig. 2.2.10). 30 SiO2 % Al2O3 % TiO2 % Fe2O3 % CaO % MgO % Alkalis % LOI. % South African sillimanite 14,43 82,97 -- 0,52 0,38 0,16 -- 1,35 Indian sillimanite 35,13 62,00 -- 0,95 0,17 0,12 -- 1,30 Indian Cyanite, pre-fired 31,61 67,68 -- 0,35 0,14 0,13 -- 0,05 French bauxit e 16,50 73,00 -- 1,5 -- -- -- 9,00 (H2O) Upper Hessian bauxite 3,20 50,60 3,0 16,4 -- -- -- 27,10 Bauxite from Belje Poljjane 24,7 55,7 3,7 0,30 0,40 -- 14,80 Sintered bauxite from Guayana 4,76 89,72 4,27 0,99 0,05 0,04 0,08 0,29 Bauxite from Katni, Jubbulpure / India 1,85 59,22 6,46 2,32 0,46 0,05 0,22 29,55 Diaspore from Missouri 6,56 72,72 3,65 2,37 -- -- 1,00 13,88 Natural corundum bis 3 93-98 -- bis 1 -- -- -- -- Fig. 2.2.10: Analysis of some high-alumina containing raw materials. This video clip taken at a raw material mine which belongs to the Imerys Group, gives you an impression about the size of such a mine. You will see an excavator vehicle collecting material from various places in the mine. The mine is a secondary deposit, in other words, the material has been already crushed by weathering. Videoclip: Deposits In the background you see earth-source with various shades of colours. These are raw materials with different chemical compositions. The decision for the material’s mixture has already to be taken in the mine, in order to get a reasonably constant composition of the raw materials to be delivered to the customers. 31 2.3 Natural ceramic raw materials 2.3.1 Kaolins and clays Silicates play an important role within natural materials and therefore their structure and characteristics are discussed first. Kaolin and clays are essentially composed of SiO4 tetrahedra. Four-free-valence silicon in a SiO4 tetrahedron is surrounded by four oxygen ions (Fig. 2.3.1.1). The silicon ion shows a positively 4-valence. Eight negative charges show the 4 oxygen ions, meaning that this SiO4 tetrahedron has, in all, 4-valences negatively charged. For neutralisation, it has either to connect to other tetrahedron or saturate these charges with cations. Fig. 2.3.1.1 demonstrates how these SiO4 tetrahedra can be linked together to form, for example, island, ring or chain silicates. If two SiO4 tetrahedra are combined by a corner, which means, by an oxygen ion, this construct contains two silicon ions and seven oxygen ions. Consequently, this 2-valence-free tetrahedron has a charge of 6-. When a ring is formed, we get the chemical formula Si3O9 with also six negative charges. Chain silicates have the formula [SiO3]2-. If negative charges are saturated by cations, we get minerals which we find in nature. 32 Fig. 2.3.1.1: Linkage of [SiO4]- tetrahedra ( = Si4+, = O2-) [1]. Island or ring silicate Chain silicate Sheet Silicate Belt silicate 33 [Si4O11]6- or [Si2O5]2- structures are formed, if the SiO4 tetrahedra are linked to sheet or belt silicates. They also have negative charges (Fig. 2.3.1.1). To become electrically neutral, cations or other positively charged structural components have to attach. Layer silicates, for example, show this effect (Fig. 2.3.1.2), when a tetrahedron layer links to an octahedron layer consisting of Al, O and OH groups. This construct is called double-single layer, because within stacking sequence every third layer shows the same geometric array as the first layer. In the lower section, where the three oxygen ions are situated, the tetrahedron layer has a negative excess charge, and in the upper section, where just one oxygen ion above the 4-valence-freesilicon ion is found, a positive excess charge. These positive excess charges in the upper part of the SiO4 tetrahedron are saturated as the octahedron layer with negative excess charge attaches to the SiO4 tetrahedron.Fig. 2.3.1.2: Double layered silicates [1]. With regard to three-layer minerals a tetrahedron layer attaches to both, the top and the bottom of an octahedron layer. These layered minerals form the basis for kaolin and clays which in the following are reviewed in detail. Water molecules can intercalate between the layers (Fig. 2.3.1.3). This intercalation of water molecules influences enormously the processing properties of clays and kaolins. Water molecules are attached by hydrogen bonds to the oxygen ions or OH groups of each layer. In case of shear stress, this slight bond causes the layers’ shifting against each other. And this leads to a certain plasticity (not related to the plasticity of metals) used for plastic forming (see chapter forming). Double single layers Stabilized octahedric double layer: (a) kaolinite- like, (b) mica-like 34 Fig. 2.3.1.3: Orientated layer of water in halloysite [3]. Bottom: Tetrahedra layer; top: Octahedra layer from the further unit. Exchange of silicon atoms in tetrahedron layers or aluminium atoms in octahedron layers brings about a variety of minerals. If a 4-valence free silicon ion is replaced by a 3-valence- free aluminium ion, a cation in the interlayer ensures valence equalisation. This cation is highly flexible in the liquid layer and can very easily be exchanged. Here, we talk about ion exchange capability (Fig. 2.3.1.4). This exchange capability can, for example, be used for water softening, as we will see later on. Anions can also be laid in or be exchanged, if the 4- valence free silicon ion is replaced by a 5-valence free ion. Fig. 2.3.1.4: Ion exchange in clay minerals [1]. 35 In addition, the number of water molecules can be varied in the inter-layers. If air humidity changes from 90 % to over 96 % to 99 % (Fig. 2.3.1.5), the clay minerals absorb more water. Water content increases from 0.4 gram water per gram dry substance at 90 % relative humidity up to 1g water/g dry substance at a relative humidity of 99 %. If humidity increases, the layer distance also increases (from 9.6 A to 16.2 A, and up to 19.5 A), what leads to a chance of the processing properties of these materials. Relative moisture [%] Total adsorption in gwater per gdry solids Average distance d [Å] between layer packages Intermediate layer water (mean number of layers) Water amount on surface of primary particles in g per gdry solids Observed bulking I. 0 0 9,6 Th e in te ns ity o f t hs in te r- fe re nc e in cr ea se s w ith th e bu lk in g pr oc es s 0 0 no bulking of the film 90 0,40 16,2 2,2 0,18 II. beginning of bulking and deformation 96 0,60 18,5 3 0,30 about 30 % of bulking 99 1,00 19,5 3,3 0,80 about 100 % bulking bulking increases up to 20 times the original film thickness III. Ad so rp tio n in co nt ac t w ith w at er about 5 The layer distance remains between 19,4 and 20 Å, whereas the (00l)- reflections disappear (00l)-reflections disappear entirely Fig. 2.3.1.5: Adsorbed water film between Na-montmorillonite layers with increasing the humidity [3]. As already mentioned, free cations are often exchanged in tetrahedra or octahedral, silicon ions in tetrahedron layers, aluminium ions in octahedron layers. The variety of minerals which can be found in nature is systematically summarised in Fig. 2.3.1.6. Mixtures of all these minerals can also be found in the nature, making their analysis quite complicated. If 3- valence free aluminium atoms occupy two octahedra each and the third octahedron remains empty, this layer succession is called dioctahedral. If 3-valence free aluminium ions in the octahedra are replaced by 2-valence-freemagnesium ions, there is a magnesium ion in each octahedron and we call this trioctahedral occupation. Than we get the mineral antigorite. If 36 additional water is given to the interlayers, receive the mineral is called halloysite. The simplest three-layer mineral is pyrophyllite. If here the aluminium ions are again replaced by magnesium ions, we get saponite. If water is added to pyrophyllite, we have montmorillonite. Fig. 2.3.1.6: Structural dependence of most important silicate minerals with layered structures [1]. If in pyrophyllite in the tetrahedron layer the 4-valence-free silicon ions are replaced by 3- valence-free aluminium ions, we find 1-valence-free cation in the interlayer for valence adjustment (muscovite etc.). On a macroscopic level, this layer structure results in flaky minerals (Fig. 2.3.1.7). Geological changes by torsion of these platelets result in minerals like nakrite or “fireclay”. At this point you can already imagine that the properties of such raw materials during transport in a pipeline are different from spherical powder particles. Clays and kaolins are very finely grained and normally further grinding is not necessary. They are just suspended in water. 37 Fig. 2.3.1.7: Order and disorder in minerals of kaolinite group [3]. 2.3.2 Feldspars In tectosilicates the SiO4 tetrahedra are interconnected in a three-dimensional way (Fig.2.3.2.1). If 2-, 3- or 4-valence-free chains are linked to the tetrahedra’s corners, corresponding networks occur. If in the SiO4 tetrahedra silicon ions are replaced by aluminium ions, electrical neutrality is re-established by incorporation of alkali or alkaline earth ions on interstitials, and this results in feldspar. Fig.2.3.2.1: Some types of tetrahedral arrangements [1]. 38 Fig.2.3.2.2: Half unit cell of lime feldspar projected on (010) [1]. The lattice structure projected on the base level is demonstrated in Fig. 2.3.2.2. The figures in this chart are multipliers with the distance from the respective oxygen or silicon ion in the basic level. Contrary to the layer minerals on interstitials, the potassium atoms are here firmly bound into the structure. A selection of feldspars found in nature is summarised in Fig. 2.3.2.3. In microcline a silicon ion was replaced in the SiO4 tetrahedron by an alumina ion. A potassium ion ensures valence adjustment (potassic feldspar). Its varying crystal modifications have different volumes, which may cause cracks during sintering of ceramic masses, as feldspar is used. If valence is adjusted with sodium ions or calcium ions, albite (sodium feldspar) and anorthite (calcium feldspar), which can also crystallise in different modifications, is formed. 39 Mineral Chemical formula Crystal system Lattice constants Density (20 °C) [g/cm³] Refractive index n n n Annotations a b c [Å] Microcline K[AlSi3O8] triclinic 8,57 12,98 7,22 90° 41' 115° 59' 87° 30' 2,57 1,514 1,518 1,521 Stable low-tempera- ture modification, ordered Sanidine K[AlSi3O8] monoclinic 8,56 13,03 7,18 – 115° 59' – 2,57 1,521 1,527 1,527 Stable high- temperature modification, disordered Albite Na[AlSi3O8] triclinic 8,14 12,79 7,16 94° 19' 116° 34' 87° 39' 2,62 1,528 1,532 1,538 Stable low-tempera- ture modification, ordered Analbite Na[AlSi3O8] triclinic 8,23 13,00 7,25 94° 03' 116° 20' 88° 09' 2,62 1,527 1,532 1,534 Instable modification, unordered Monalbite Na[AlSi3O8] monoclinic 7,25 12,98 6,41– 116° 07' – 1,523 1,528 1,529 stable high- temperature modification, unordered Anorthite Ca[Al2Si2O8] triclinic 8,18 12,88 14,17 93° 10' 115° 51' 91° 13' 2,77 1,576 1,583 1,589 ordered Celsian Ba[Al2Si2O8] monoclinic 8,65 13,13 14,60 – 115° 02' – 3,8 1,587 1,593 1,600 Fig. 2.3.2.3: Properties of some feldspars [1]. 2.3.3 Quartzite and sands Quartz also originates from a three-dimensional linkage of SiO4 tetrahedra. The figures shown in Fig. 2.3.3.1 describe again the distance from the base level, for example, c axis by a factor of 0.33. The SiO4 tetrahedra unscrew themselves spirally from the base level. Fig. 2.3.3.1: Unit cells of low quartz (a) and high quartz (b) projected on the plan (0001) [1]. 40 Fig. 2.3.3.2: Phase transformation of SiO2 with temperature. SiO2 can be found in different crystallographic modifications (Fig. 2.3.3.2). Low temperature modification of quartz (β-quartz) into α-quartz takes place at a temperature of 573° C. Here just a marginal shift of silicon and oxygen ions can be observed. The temperature of 870°C is known as the high temperature modification of quartz to tridymite. In this case, new bonds are formed. Therefore this transformation does not happen very quickly, while the transformation from α-quartz into β-quartz is quick and unavoidable. Reconversion of tridymite into quartz can be prevented, if it is cooled very quickly, leaving no time for the structure to reconvert. Further transformations are related to α-cristobalite at 1,470°C, and SiO2 melting at 1,713°C. Such transformations cause tremendous problems for the sintering of ceramic products, because they are partly combined with major volume changes. Quartz inversion at 573°C leads to a volume expansion of 0.8 %. This expansion of the volume may indeed cause cracks in the porcelain during the cooling stages, after sintering. This problem is accentuated during the quartz transformation into cristobalite or tridymite. This is accompanied by a volume expansion of more than 15 % (Fig.2.3.3.3), which after sintering causes stresses in the structure and destructs the components during cooling. 41 temperature s p e c if ic v o lu m e Fig. 2.3.3.3: Dependence of the specific volume on the temperature for quartz, cristobalite and tridymite [1]. Transformation temperature may change as chemical impurities are added or when the order varies (Fig. 2.3.3.4). 42 Fig. 2.3.3.4: Dilatometer curves of cristobalite with distinct degree of orientation in comparison to well-oriented tridymite [1]. If quartz after adding 5 weight % of feldspar is exposed to a temperature treatment of 1,300°C and a holding time of 14 hours, a structure which consists of 40 % cristobalite and 60 % quartz (Fig. 2.3.3.5) can be obtained. If 2wt% CaO is added, at 1,300°C and 14 hours holding time, almost 100% of quartz can be found in the sintered product. At 1,400°C and a very short holding time, tridymite is formed, and at a slightly higher temperature also cristobalite. The situation changes even further if sodium is added. This means that the transformation temperature changes depending on the raw material used and their level of chemical contamination. Transformation velocities can be influenced by appropriate temperature time curves (holding times), so that stresses during cooling can be avoided. U = interval of transformation Δ = hysteresis area 43 Fig. 2.3.3.5: Influence of additives on the temperature and time of quartz transformation [1]. Materials containing SiO2 are often applied as firing auxiliaries when ceramic materials are sintered under increased pressure (hot pressing or hot isostatic pressing). In this case, pressure induced crystal transformations (Fig. 2.3.3.6) is expected. These transformations are also combined with volume transformations and may cause the components’ damage. Fig. 2.3.3.6: Pressure-temperature phase diagram for SiO2 [1]. 44 2.3.4 Binary and ternary silicates, high alumina containing raw materials Some characteristics of binary silicates are described in Fig. 2.3.4.1. Negative charges of SiO4 tetrahedra can be saturated, for example, by magnesium ions (forsterite), iron ions (fayalite) or by zirconium ions (zircon). The formation of enstatite takes place if, for example, two SiO4 tetrahedra are combined with each other and valence adjustment is made by magnesium ions. If silicon ions are replaced by aluminium ions and valence adjustment is made with aluminium ions, sillimanite or andalusite/mullite is formed. The systematic formation of silicates is presented in Fig. 2.3.4.2. Mineral Chemical formula Crystal system Lattice constants Density (20 °C) Refractive index Linear coefficient of expansion [10 -6 K -1 ] a b c pm [g/cm³] n n n Forsterite Mg2[SiO4] orthorhom bic 598 478 1025 3,21 1,636 1,651 1,669 20/1000: 11 Fayalite Fe2[SiO4] orthorhom bic 617 481 1061 4,35 1,824 1,864 1,875 Zircon Zr[SiO4] tetragonal 659 – 594 4,6 1,94 1,99 20/1000: 4,5 Enstatite Mg2[SiO6] orthorhom bic 1822 881 520 3,18 1,650 1,653 1,658 Protoenstatite Mg2[SiO6] orthorhom bic 925 874 532 3,10 similar to Enstatite 20/1000: 11 Clinoenstatite Mg2[SiO6] monoclinic 961 882 520 71° 40' 3,18 1,651 1,654 1,660 20/600: 8,9 Wollastonite Ca3[Si3O9] triclinic 794 732 707 90° 02' 95° 22' 103° 26' 2,92 1,620 1,632 1,634 20/800: 12 Sillimanite Al[AlSiO5] orthorhom bic 748 767 577 3,25 1,657 1,658 1,677 25/300: 3,2 25/600: 4,6 25/900: 6,0 Andalusite Al2[O/SiO4] orthorhom bic 779 790 556 3,14 1,632 1,638 1,643 25/300: 8,7 25/600: 10,6 25/900: 11,9 Kyanite Al2[O/SiO4] triclinic 710 774 557 3,67 1,717 1,722 1,729 25/300: 8,8 25/600: 9,2 25/900: 9,2 Mullite (3:2) Al[Al1,25Si0,7 5O4,875] orthorhom bic 754 767 283 3,16 1,642 1,644 1,654 25/1000: 4,5 Mullite (2:1) Al[Al1,4Si0,6 O4,8] orthorhom bic 757 768 289 3,17 1,650 – 1,663 Fig. 2.3.4.1: Properties of some binary silicates [1]. 45 Type Shape Dimensionality Silicate anion O /Si ratio Example Name Chemical formula Tetrahedrons single double 0 0 [SiO4] 4- [Si2O7] 6- 4,0 3,5 Forsterite Rankinite Mg2[SiO4] Ca3[Si2O7] Rings 3-fold ring, single 6-fold ring, single 6-fold ring, double 0 0 0 [Si3O9] 6- [Si6O18] 12- [Si12O30] 12- 3,0 3,0 2,5 Benitoite Beryllite Milarite BaTi[Si3O9] Al2Be3[Si6O18] KCa2AlBe2[Si12O30]• ½ H2O Chains single double 1 1 [SiO3] 2- [Si4O11] 6- 3,0 2,75 Enstatite Tremolite Mg[SiO3] Ca2Mg5[Si4O11]2(OH)2 Sheets single 2 [Si4O10] 4- 2,5 Kaolinite Al4[Si4O10](OH)8 Frameworks – 3 [SiO2] 2,0 Quartz SiO2 Fig. 2.3.4.2: Systematic formation of silicates [1]. Mineral Chemical formula Crystal system Lattice constants Density (20 °C) Refractiv e index Annotations a b c [pm] [g/cm³] n n n Low nepheline Na[AlSiO4] hexagonal 1001 841 2,62 1,533 1,537 at 850 °C high temperature modification High nepheline Na[AlSiO4] orthorhombic 1020 1760 850 2,47 at 1254 °C high carnegieite Low carnegieite Na[AlSiO4] triclinic 2,51 1,509 1,514 1,514 at 690 °C high temperature modification High carnegieite Na[AlSiO4] cubic 732 2,34 1,510 Low leucite K[AlSi2O6] tetragonal 1304 1385 2,47 1,508 1,509 at 620 °C high temperature modification High leucite K[AlSi2O6] cubic 1343 2,47 1,509 Kaliophilite K[AlSiO4] hexagonal 2706 861 2,60 1,532 1,527 metastable Kalsilite K[AlSiO4] hexagonal 518 869 2,59 1,542 1,539 metastable synthetic K[AlSiO4] orthorhombi c 901 1567 857 2,60 1,528 1,536 1,537 stable Petalite Li[AlSi4O10] monoclinic 1176 514 752 112° 24' 2,42 1,504 1,510 1,516 stable only < 900 °C Low spodumene Li[AlSi2O6] monoclinic 952 832 525 110° 28' 3,15 1,72 at 700 °C high temperature modification High spodumene Li[AlSi2O6] orthorhombi c 1838 1061 1068 2,44 1,52 Low eucryptite Li[AlSiO4] trigonal 1353 904 2,67 1,572 1,587 at 970 °C high temperature modification High eucryptite Li[AlSiO4] hexagonal 524 1113 2,33 1,524 1,520 Fig. 2.3.4.3: Properties of some ternary aluminium silicates [1]. 46 In ternary aluminum silicates the silicon atom within SiO4 tetrahedron is replaced by an aluminum atom. Valence adjustment occurs by alkali or alkaline earth ions like, for example, sodium ions (nepheline) or potassium ions (leucite) etc. (Fig. 2.3.4.3). The feldspars already mentioned belong to this group of materials. Fig. 2.3.4.4: Unit cells of sillimanite (a) and (b) mullite projected on (001) [1]. Fig. 2.3.4.5: Dependence of lattice constants a and b of sillimanite and mullite from Al2O3- content [1]. 47 Sillimanite and mullite (Fig. 2.3.4.4) are Al2O3-SiO2 mixed crystals which are characterized by slight shifts of the lattice spacing in the crystal structure. In Fig. 2.3.4.4 tetrahedral coordinated Si atoms and AI ions in octahedral coordination can be found. So that, aluminum ions are surrounded by six oxygen ions while Si ions by four. The lattice constant changes depending on the content of alumina (Fig. 2.3.4.5). Mineral Crystal system Specific weight before firing Increase in Volume [%] Initial tempera- ture of mullite- crystallization [°C] Cyanite triclinic 3,5 to 3,6 16 to 18 1325 Andalusite rhombic 3,1 to 3,2 3 to 6 1350 Sillimanite rhombic 3,23 to 3,25 7 to 8 1530 Fig. 2.3.4.6: Transformation of sillimanite group materials into mullite via heating [3]. Fig. 2.3.4.7: Phase diagram for the system SiO2-ZrO2 [adapted from R.F. Geller and S.M. Lang]. 48 Further minerals of the sillimanite group are cyanite, andalusite and sillimanite (Fig. 2.3.4.6). Zirconia and silica form a peritectic melting compound in zirconium silicate (Fig. 2.3.4.7). Peritectic melting point means that above 1,775°C there is equilibrium between a melt and solid solutions, in which the melt portion increases by increasing the temperature. Above 2,600°C the solid solution is completely melted. Name Chemical formula Amount of Al2O3 (theoretical) [%] Amount of water (theoretical) [%] Specific weight Shrinkage [%] Transformation temperature [°C] transforms into Diaspor -Al2O3 • H2O 85,0 15 3,36 – 450 -Al2O3 Boehmite -Al2O3 • H2O 85,0 15 3,01 33 280 -Al2O3 Hydrargillite (Gibbsite) -Al2O3 • 3 H2O 64,5 34,6 2,3 bis 2,4 60 150 Boehmite Bayerite Al2O3 • 3 H2O 64,5 34,6 – 60 150 Boehmite Fig. 2.3.4.8: Properties and heating behavior of hydrated alumina [3]. Diaspor, boehmite, hydrargillite und bayerite (Fig. 2.3.4.8) belong to these high alumina containing raw materials. Al2O3 can be produced from such Al-hydroxides by calcination. 2.4 Synthetic ceramic raw materials 2.4.1 Silicates Depending on the charge, natural raw materials may show slightly differing chemical compositions or different grain size distributions. This makes processing in manufacturing plants occasionally difficult. And this is the reason for experiments to produce silicates synthetically. Fig. 2.4.1.1: Production of synthetic, amorphous and crystalline sodium dissilicates. 49 Fig. 2.4.1.2: Production of SKS-6 and SKS-6-Co- Granulate at Hoechst. Fig. 2.4.1.3: Structure from amorphous and crystalline synthetic sodium-dissilicate (Hoechst). For the production of synthetic silicates SiO2 is hydrothermally solubilised with caustic soda (Fig. 2.4.1.1 and Fig.2.4.1.2). This leads to an aqueous water-glass solution which is spray dried. The result is an amorphous disilicate which after further dewatering is transformed to a crystalline disilicate. The crystal structures consist of tetrahedron and octahedron layers with water in the interlayers, just like the natural layer silicates (Fig. 2.4.1.3). 50 As these synthetic silicates are stirred in water, calcium ions from the water get integrated into the interlayers and the water is softened (Fig.2.4.1.4). Compared to natural layer silicates, the pureness of these synthetic materials is an advantage (fig.2.4.1.6). The content of alumina and silica is comparable. But there are clear differences with regard to the iron content which is particularly responsible for the raw materials’ discoloration. Synthetic kaolins could never establish in ceramic industries because of their high production costs. But they are used, for example, for water softening within detergents industries. Fig. 2.4.1.4: Different properties form SKS-6 (Hoechst). Elements Natural Synthetic Al2O3 30,1% 31,7% SiO2 47,7% 48,1% Na2O 0,08% 0,3% K2O 1,2% 0,01% MgO - 0,03% Fe2O3 0,76% 0,03% H2O (110°C) n.d. 5,6% Weight loss 14,4% 17,6% Fig. 2.4.1.5: Chemical composition of synthetic and natural kaolins. Besides the type of water softening element, the effectiveness of the builder in removing these elements is also important. Here, the difference in effectiveness especially at higher additions of amorphous or crystalline di-silicates. 51 2.4.2 Oxides All oxides of the periodic table elements belong to ceramic materials. Fig. 2.4.2.1 shows a small selection of the most important oxides which are very attractive due to their field of application. Among them we find beryllium oxide, magnesium oxide, calcium oxide, alumina, yttria, zirconia as well as hafnium and thorium oxide, which have extremely high melting temperatures. Ceramic oxide materials are characterized by their ionic bond and therefore show no electronic conductivity. This explains the high specific electrical resistance. a stabilized, only for support Fig. 2.4.2.1: Properties of high-melting oxides [1]. 52 Fig. 2.4.2.2: Illustration of production of Al2O3 from bauxite according to the Bayer process. Exemplary for alumina, Fig. 2.4.2.2 shows how this synthetic oxide is made from natural raw materials. Bauxite as a natural raw material is the basic material for the production of aluminum oxide. Bauxite is a mixture of different aluminum hydroxides, contaminated with iron hydroxides, silicates and titanium oxides. First the raw materials are grinded to a grain size of < 1 mm. Then they are processed with sodium hydroxide in an autoclave at a pressure of 40 bars and a temperature of about 250°C. A sodium aluminatesolution is formed dissolving the alumina hydrates as aluminates. Iron oxide, titanium oxide and SiO2 remain undissolved. This so-called red mud (red coloration caused by iron hydroxide) can be separated by filtration from the sodium aluminate. Aluminum hydroxide seed crystals are now dispersed in the aluminate solution and aluminum hydroxide again crystallizes and can be separated by filtration from the sodium hydroxide. This aluminum hydroxide is transformed into aluminum oxide by a thermal treatment in a rotary kiln. Because of the sodium hydroxide and the high temperatures this is a relatively crucial production process, if aluminum oxide is produced in ton sizes. 53 Fig. 2.4.2.3: Dilatometer curve of ZrO2 [1]. Zirconia is another interesting oxide. ZrO2 shows particularly problematic properties. Depending on temperature, it changes to various crystal modifications, partly causing big volume changes (Fig. 2.4.2.3). At slightly above 1,000°C the low temperature modification of ZrO2 changes to high temperature modification. This may, for instance, occur during sintering. Reconversion happens during cooling. Normally this occurs at slightly lower temperatures, and a hysteresis loop is shown. Broadness of the hysteresis loop depends on the cooling rate. The enormous volume changes cause stresses and cracks, and this is the reason why components cannot be made of pure ZrO2. This crystal transformation can be prevented, if almost 20 mol% calcium are added, which causes a solid solution formation. Such volume transformations occur in many ceramic materials and have to be considered for the production of ceramic products. To achieve a product free from cracks, these crystal transformations must be avoided. 54 2.4.3 Non-oxide materials Non-oxide ceramic materials comprise the elements silicon, nitrogen, carbon, boron and aluminium. The chemical compounds herein are: BN, Si3N4, AIN, SiC, B4C, SiB4(6). The elements of which the mentioned compounds result, stand within the periodic table all relatively far to the right. This means that their electronic configuration in the outer orbital has been already relatively completed. Therefore, the different elements provide each other with electrons for covalent bonds without electronic conductivity, what is a typical characteristic of the ceramic materials. Further left on the periodic table, the elements like titanium, hafnium or zircon which also form carbides with carbon or nitrides with nitrogen can be found. Due to the presence of metallic bonds and their electronic conductivity, they are no far considered as ceramics. Fig. 2.4.3.1: Production of carbides. The raw materials necessary for the production of these ceramic materials are not found in nature, but made synthetically. Fig. 2.4.3.1 shows carbides as an example for these synthesis pathways. Carbides can be made from the elements, by carbothermal reduction or by chemical vapour deposition. With regard to the elements, silicon reacts at appropriate temperatures with carbon to SiC. Such production of silicon carbide in ton sizes is too expensive. Far more often, SiC is produced by a carbothermal reduction in which quartz 55 powder (sand) reacts with carbon to SiC and CO2. SiC can be then produced in large-scale productions. If for scientific purposes there is a need to use high-purity SiC, the SiC should be produced from the gas phase by allowing, for example, silicon tetrachloride to react with CH4 to SiC and HCl. Fig. 2.4.3.2: Production of nitrides. Production of nitrides can be made similarly (Fig. 2.4.3.2): via the elements, by carbothermal reduction and from the gas phase. Via the elements: silicon reacts with nitrogen to Si3N4. Via carbothermal reduction: quartz powder reacts with ammonia to Si3N4 and H2. From the gas phase: silicon tetrachloride reacts with NH3 to silicon nitride and hydrochloric acid (SiCl4+NH3-Si3N4+HCl). The same applies for borides and silicides. For shaping processes these synthetic materials have to be grinded and crushed, purified and mixed to ceramic masses. 56 2.5 Organic raw materials Fig. 2.5.1 shows the mechanism of action of different organic additives in ceramic masses. More than 95 % of the ceramic materials are processed as suspensions either during their preparation or during shaping. In many cases the required solvent is water, but organic solvents can be also used. Binders, wetting agent, defoaming or conservation agents are normally added to the suspensions. Binders increase the body’s green strength often necessary for the transport of the parts after shaping to the kilns. The binders can be modified by adding plasticizers or softeners. Releasing and anti-blocking agents reduce the friction in compression moulds and increase the powder consolidation. So, normally ceramic masses consist of ceramic powders and a large number of organic additives. Fig. 2.5.1: Function of the different components in ceramic masses. In Fig. 2.5.2 the variety of organic additives for different shaping processes is shown. Prior to the sinter, the additives must be burn out or must be regained by condensation. 57 Dry Pressing Polyacrylates, huminates Dispersants Polyvinyl alcohols, tyloses, waxes Binder Polyethylene glycols Flow agent Slip Casting Polyacrylates, fish oil Dispersants Polyvinyl alcohols, modified Binder Polysaccharides, polyvinyl butyrals Water, trichlor ethylene, ethanol Dispersing agents Tape Casting Oleates, polyacrylates, Fish oil, Dispersants Phosphoric acid ester Polyvinyl butyrals, polyvinyl alcohols, Binder Acrylic resin, plastic dispersions Phthalates, polyethylene glycols, Softener Phosphoric acid ester Toluol, trichloroethylene, methanol, Dispersion agents Ethanol, methyl isobutyl ketone, water Injection moulding Oleates, polyacrylates Dispersants Polyethylene, polystyrene, waxes Binder (Dispersants) Polyethylene glycols Flow agents Caster Oil Porosity inducing agents Extrusion Polyacrylates Dispersants Polyvinyl alcohols, tyloses Binder Polyethylene glycols, glycerine Flow agents Fig. 2.5.2: Function of organic additives in dependence of the processing technology. Fig. 2.5.3 shows additives for aqueous and non-aqueous tape casting. There are different binders, plasticizers, condensers and wetting agents depending on the nature of the solvent, if organic solvents and water based solvents. 58 Solvents Binders Plasticizers Deflocculants Wetting agents Non-aqueous Acetone Ethyl alcohol Benzole Bromochlorome- thane Butanol Diacetone Ethanol Isopropanol Methylic isobutylketone Toluol Trichlorethylene Xylol Celluloseacetate Butyrate resign Nitrocellulose Petroleum resign Polyethylene Polyacrylate ester Polymethylmetha- crylate Polyvinyl-alcohol Polyvinyl-butyral resign Polyvinyl-chloride Butylbenzylphtalate Butylstearate Dibutylphtalate Dimethylphtalate Methylabietate Mixed Phtalate esters (Hexal-, octyldecylalcohol) Polyethylenglycol Polyalkylenglycol Polyalkylenglycol derivates (Triethylenglycol- hexoat) Trikresylphosphate Fatty acids (Glyceryl- tri-oleate) Natural Fish oils (Menhaden) Synthetics (Benzene sulfonic acids) Alkylarylpolyether alcohol Polyethylenglycole- thylether Athylphenylglycol Polyoxyethylenace- tate Polyoxethylenesther Aqueous Water (with antifoaming agents based on resigns) Acrylic polymer Acrylic polymer emulsion Ethylenoxide polymer Hydroxyethylencel- lulose Methylcellulose PolyvinylalcoholTRIS. Isocyaminate Resign based sliding additives Butylbenzylphtalate Ethyltoluolsulfona- mide Glycerine Polyalcylenglycol Triethylenglycol Tri-N-butylphos- phate Complex vitreous phosphates Condensed arylic sulfoic acid Natural sodium salt Non-ionic octyl- phenoxyethanol Fig. 2.5.3: Additives for tape casting of aqueous and non-aqueous slurries. Generally, binders or plasticizers are used to increase the interval of ignition and avoid crack formation during burn out. This makes the whole system very complex and the understanding of the ignition products is extremely difficult. To understand the coupling behaviour of organic additives on surfaces of oxidic ceramic particles, we observe the surface of an aluminium oxide particle (Fig. 2.5.4). Ceramic powders normally have a high specific surface and show electric charges in aqueous suspensions. Such electric charges may be explained as follows: oxides show unsaturated valences at their surface due to the incomplete coordination of atoms. When this oxide surface gets in contact with water the surface becomes hydrated. Once these particles are linked to air humidity their surfaces hydrate. The number of the developing -OH groups at the surface depends on the number of oxygen atoms at the surface, and this in turn depends on the crystal structure. With regard to aluminium oxide the oxygen ions are saturated with hydrogen at a pH value of 9, this means that at this pH value the particles are electrically neutral. 59 Fig. 2.5.4: Creation of a hydrated Al2O3 particle surface and its reaction in acidic and basic solutions (schematic). As the pH value is shifted to an alkaline value (by adding -OH) an OH group combines with a hydrogen ion, adsorbs water and a negatively charged surface. When H3O+ is added, water regenerates and an additional proton adsorbs onto the surface, therefore a positively charged surface is formed. According to the pH value ceramic particles’ surfaces can be charged positively or negatively. Organic molecules can be coupled to these positively or negatively charged surfaces. In Fig. 2.5.5 oleates adsorbed into an aluminium oxide particle surface is shown. These long-chain molecules prevent particles to agglomerate into a suspension. This is called steric stabilisation. Fig 2.5.6 shows schematically the steric constraint of the powder particles’ approach to a suspension by adsorbing long-chain molecules. 60 Fig. 2.5.5: Oleate coupling to Al2O3 surface (schematic). Fig.2.5.6: Steric hindrance due to the approach of powder particles in a suspension by long- chain molecules settled down (schematic). 61 When negatively or positively charged, surfaces reject each other. It is referred to an electrostatic stabilisation (Fig. 2.5.7). Ceramic particles do not sediment in well dispersed ceramic suspensions, the so-called stabilized suspension. (a) (b) Fig. 2.5.7: Electrostatic (a) and steric (b) stabilisation of colloid dispersions and emulsions [G. Lagaly, Universität Kiel]. In most cases, organic additives are not an exactly defined chemical product. They often show a broad distribution of their molecule chain’s length. This can lead to changes during processing. Therefore, the so-called di-block copolymers were developed (Fig. 2.5.8); the length of the adherent and the stabiliser blocks can be variable and then the characteristics could be systematically investigated. Attempts have also been made to understand which crack products occur if these additives are burned out under inert gas or oxygen atmosphere. 62 Fig. 2.5.8: Different structures in di-block copolymers. [G. Wegner, MPI für Polymerforschung,1996]. Fig. 2.5.9 and Fig. 2.5.10 show the influence of the additives on the viscosity of slurries and the stability of green compacts prepared from this suspensions. If the water content of a suspension is low, the costs for drying are also low. After the shaping, any added organic has to be burned out. Burning-out times vary considerably according to the process of shaping, because different quantities of organics have to be added. 63 Fig. 2.5.9: Change in viscosities of 64.4% commercial available slurries of porcelain mass due to the presence of different additives. 1 = without additives 2 = Cellulose (CMC) (yield stress: 56 Pa) 3 = Ethylene · vinyl acetate · copolymer (EVAC) 4 = Polysaccharide (PS) 5 = EVAC/PS (2:1) 6 = Polyvinyl alcohol (4-88) 7 = Polyacrylate 8 = Polyacrylate dispersion Bayceram® VP PN PB 4305 9 = Bayceram® VP PN PB 4305/ Bayceram® VP PN PB 4306/ Bayceram® VP PN PB 4307 1:1:1 10 = Bayceram® VP PN PB 4305/ Bayceram® VP PN PB 4306/ Bayceram® VP PN PB 4307 5:1:1 11 = Bayceram® VP PN PB 4305/ Bayceram® VP PN PB 4306/ (0,8% additive) Bayceram® VP PN PB 4307 5:2:1 64 Fig. 2.5.10: Influence of the humidity content on the green strength of spray dried granulates due to the addition of Bayceram®. Fig 2.5.11 shows a typical curve of the mass loss of organic additives depending on the temperature. Maximal burn out rate can range up to temperatures of 800°C and sometimes even beyond that. If just one binding agent would be used, and if this burns away at, for example, exactly 500 °C, burning-out at this temperature would happen very quickly. 65 Fig. 2.5.11: Dependence of the mass loss of organic additives with the temperature. During the solid-liquid-gaseous transition, high volume changes occur, what could lead to crack formation in the components. Therefore different organic additives are mixed in order to conduct to different burning-out temperatures. The burn-out of additives added to oxide ceramic materials can take place in air, while in non-oxide materials some oxidation at temperatures between 800°C and 900°C can take place. Therefore binders are sometimes burned-out in inert gas atmosphere which can be observed through a displacement in their burning-out curves. In Fig. 2.5.12, for example, polyvinylbutiral, PVB, a binder used for tape casting was burned-out under air and under nitrogen. The maximum burn-out rate shifts from about 300°C to about 330°C. In practice, holding times are taken at such temperatures to reduce the burning-out velocity. 66 Fig. 2.5.12: Weight loss of polyvinylbutiral during heating under air and nitrogen atmosphere [E.Wessely, FH Nürnberg]. The number and amount of additives to be added varies according to the shaping procedure. If for dry pressing the bodies contain 1 vol.% to 10 vol.% of organic additives, injection moulding masses contain about 40 vol.% of organic additives. A dry pressing article with an organic content of 5 vol.% can be burned-out within a few hours. This is often carried out in the aggregates where sintering also takes place. Burning- out of an injection moulded article with an organic content of 40 vol.% can last up to two weeks, depending on the component’s complexity and thickness of its body, and is carried out in separate burning-out aggregates (Fig. 2.5.13). 67 Fig. 2.5.13: Weight loss during burn out of organics in dry pressed and injection moulded bodies. 2.6 Raw material preparation In nature, natural raw materials are not found within a narrow spectrum of grain size and only on rare occasions occur as a pure mineral. Grain size distributions are often represented as shown in Fig. 2.6.1. In this case, the sum of residues is plotted as a function
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